Human feet support the body at three positions, thenar eminences (joints of big toes), hypothenar eminences (joints of little toes), and heels while the human stands up straight. The human foot has three arch structures bridged across these positions. The arch structures are consisted of muscles and ligaments. These arch structures serve as a cushion to absorb an impact applied to the three positions. Specifically, the three arch structures are formed of a medial longitudinal arch, a lateral longitudinal arch, and a transverse arch. The medial longitudinal arch is bridged across the thenar eminence and the heel and forms a plantar arch. The lateral longitudinal arch is bridged across the hypothenar eminence and the heel. The transverse arch is bridged across the thenar eminence and the hypothenar eminence.
Nowadays, increasingly more people suffer from fallen transverse arch or the arch warped in the opposite direction (downward) due to wearing high-heeled footwear or similar footwear, which causes an excessive load to be applied to the toes. Thus, the person whose shape of the transverse arch has collapsed cannot sufficiently absorb an impact applied to the feet and therefore easily gets tired. Additionally, the person is likely to develop a hallux valgus, a callus, a clavus, or a similar symptom. To prevent the problem caused by collapsing of the transverse arch, there has been proposed footwear and a sole insert whose foot contact surfaces are bulged into the shape of the transverse arch to support the transverse arch during wearing (see Patent Literatures 1 to 3).